Metal bump for a thermal compression bond and method for making same

ABSTRACT

A structure for bonding to a conductive pad on a semiconductor substrate is described. The structure comprises a glassy passivating layer with a thickness of at least 3 microns deposited over the conductive pad. The passivating layer defined an aperture which exposes a portion of the conductive pad. A metal bump covers the portion of the conductive pad exposed in the aperture and further extends over the edges of the glassy passivating layer so as to form a seal between the conductive pad and the glassy passivating layer. A subsequent thermal compression bonding operation on such structure does not cause fractures in the glassy passivating layer due to its thickness.

This is a divisional of copending application Ser. No. 07/640,658, filedon Jan. 14, 1991.

FIELD OF THE INVENTION

This invention relates to metal lead bonding to semiconductorstructures, and more particularly to a metal bump structure particularlyadapted to thermal compression bonding between a beam lead and asemiconductor chip's contact land.

BACKGROUND OF THE INVENTION

The use of metal bumps to bond external metal beam leads to contactpatterns on semiconductor chips is well known in the art Beam leadbonding, flip-chip bonding, and tape automated bonding (TAB), forexample, comprise examples of semiconductor chip mounting processes thatutilize metal bumps formed over selected regions of the wiring patternon a chip. In thermal compression bonding, a combination of heat andphysical pressure is used to bond metal bumps on the chip to externalmetal connections. Those external connections can be provided, forexample, on an external chip or a flexible tape, and positioned so as tobe in facing relationship to the metal bumps.

Referring to FIG. 1 a prior art bump/chip structure is shown which hasbeen found to present potential corrosion problems leading to chipmalfunction. A semiconductor substrate 10 has formed thereon aconductive contact land 12 to which a metal bump 14 is affixed.

A glassy, passivating layer 16 overlays the surface of substrate 10.During the processing of substrate 10, an opening is etched in layer 16by placing a mask on its upper surface, followed by an etch. The etchedopening has generally been made somewhat larger in cross section thanbump 14 in order to assure that the bump, when it is deposited, isseated squarely on contact land 12.

The composition of bump 14 includes a bonding layer 20 of chromium onwhich an aluminum pedestal 22 was deposited. Next, an additional layerof chromium 24 is laid down upon the uppermost surface of aluminumpedestal 22, followed by layers of copper 26 and gold 28.

As a consequence of this structure exposed collar areas 18 were createdwhere corrosion could attack the uppermost surface of contact land 12.While attempts have been made to passivate collar region 18 through theapplication of polymeric overcoats, the results have not been totallysatisfactory.

Other prior art showing methods for forming bumps on conductive lands onsemiconductors can be found in U.S. Pat. Nos. 4,042,954, 4,427,715, and3,874,072. In U.S. Pat. No. 4,042,954 to Harris there is shown a methodfor forming metal bumps which utilizes a multilayer transition structureof Cr, AlCr, Cr and Au. This transition structure is used to connect anickel-under-copper bump to an aluminum metal pattern on a semiconductorchip. The aluminum metal pattern contacts selected regions on the chipthrough an SiO₂ passivating layer.

In U.S. Pat. No. 4,427,715 to Harris, another method is shown forforming metal bumps wherein a bump is centered over a pad so as to covera window in an intermediate passivating layer. The positioning and sizeof the bump is selected relative to the pads such that during thermalcompression bonding, the periphery of the bump does not extend over theperiphery of the pad. This arrangement is purported to prevent failurescaused by cracking in the passivating layer.

In U.S. Pat. No. 3,874,072 to Rose et al., a method is shown for formingmushroom-shaped metal bumps which incorporates multiple layers ofvarying metals. Briefly, a nickel mushroom cap is bonded to an aluminumlayer through intervening nickel, chromium layers respectively. Thealuminum layer is deposited on an aluminum pad through a window in athin glass passivating layer. Thin layers of gold tin and gold areformed sequentially over the nickel cap.

The above patents suffer from a number of disadvantages. Some do notanticipate the corrosion problems which may occur due to either a lackof a good bond between an aluminum bump and an underlying insulatingstructure or from exposed contact land metallurgy. Others do notanticipate that the misalignment of a bump can cause cracking of anunderlying passivating layer.

In summary, there is a need in the art for a metal bump process, usefulin thermal compression bonding which is both inexpensive to manufacture,inhibits corrosion due to exposed metallurgy, and maintains optimumsub-bump structures.

Accordingly, it is an object of this invention to provide a new andimproved metal bump for use in thermal compression bonding processes onsemiconductor chips.

It is another object of this invention to provide an improved method formanufacturing interconnecting metal bump structures wherein passivationlayer cracking is avoided.

It is still another object of this invention to provide a metal bumpstructure that does not leave regions which expose underlying contactmetallurgy which is corrosion-sensitive.

It is yet another object of this invention is to provide a metalbump/semiconductor structure wherein a glassy passivating layer may beemployed.

SUMMARY

A structure for bonding to a conductive pad on a semiconductor substrateis described. The structure comprises a glassy passivating layer with athickness of at least 3 microns deposited over the conductive pad. Thepassivating layer defines an aperture which exposes a portion of theconductive pad. A metal bump covers the portion of the conductive padexposed in the aperture and further extends over the edges of the glassypassivating layer so as to form a seal between the conductive pad andthe glassy passivating layer. A subsequent thermal compression bondingoperation on such structure does not cause fractures in the glassypassivating layer due to its thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a prior art conductive bump/semiconductorstructure.

FIG. 2 is a sectional view of a preferred embodiment of the invention.

FIG. 3 is a sectional view of the conductive bump/semiconductorstructure shown in FIG. 2, after thermal compression bonding.

FIG. 4 is a plan view of a section of a semiconductor chip whichillustrates an active circuit-free area surrounding the bump regions.

DETAILED DESCRIPTION OF THE INVENTION

As techniques for the fabrication of monolithic semiconductor devicesare well known, it is assumed that the production of semiconductordevices in substrate 10 has already been accomplished and that pads 12have been emplaced around the periphery of the chip. In FIG. 2, a crosssection of a single pad 12 is shown emplaced on semiconductor chip 10.While the metallurgy of pad 12 may be comprised of a number of metalcomponents, it is preferred that it comprise an aluminum/copper mixturethat is evaporated onto the surface of chip 10.

Subsequent to the deposition of pad 12, a passivating layer ofinsulating quartz 16 is deposited over the entire upper surface ofsubstrate 10. The thickness range of quartz layer 16 is preferablyapproximately 3 to 4 microns but may be greater. The preferred methodfor depositing quartz layer 16 is via sputtering.

An opening or window is made in quartz layer 16 over pads 12, bysuperimposing a mask over quartz layer 16 and then subsequently etchingan opening to the surface of pad 12. The openings through quartz layer16 are called via-holes and are placed around the periphery of the chipin order to facilitate subsequent thermal compression bondings to innerleads of a TAB tape.

Subsequently, a molybdenum mask having openings around its peripherythat are aligned with the via holes is emplaced over quartz passivatinglayer 16. The holes in the mask are sufficiently wide to not only exposethe via holes but also shoulder areas 30 and 32 which surround each viahole. An aluminum bump structure 14 is then deposited through themolybdenum mask via a series of evaporation steps. Aluminum bumpstructure 14 comprises an adhesion layer of chromium 34 which extendsnot only over the exposed portion of land 12, but also up the side wallsof the via hole and over shoulder areas 30 and 32. Chromium layer 34 ispreferably approximately 1250 angstroms thick and adheres well to boththe metallurgy of land 12 as well as covered areas of quartz layer 16.

Subsequently, an aluminum bump pedestal 36 is deposited on chromiumlayer 34 to a thickness of approximately 18.65 microns +/-2.5 microns,followed by an additional layer of chromium 38. Nexr, a thin compositeadhesion-promoting layer of chromium/copper 40 is deposited on chromiumlayer 38 and is followed by an approximately one micron layer of copper42. Finally, a 4500 Angstrom layer of gold 44 is deposited on theuppermost surface of the pedestal and forms the bonding surface for thebump.

Layer 34 of chromium provides enhanced adhesion between aluminum bump 36and the metallurgy of pad 12 and quartz layer 16. Thus, during thermalcompression bonding or subsequent thermal cycling, the bumps areprevented from shearing-off through the adhesive properties of chromiuminterlayer 34. Chrome layer 38 acts as a diffusion barrier betweenaluminum bump 36 and copper layer 42 so that aluminum/copper alloyformation is prevented which is potentially corrodible. Finally, theintermixing layer of copper and chromium in layer 40 promotes theadhesion of copper onto the chromium layer.

After bump deposition, chip 10 is placed on an anvil and has itscontacts aligned with the inner leads of a TAB tape. Generally, suchleads are comprised of copper which have been plated with a thin layerof gold. Then, a thermode is brought down upon the inner leads, pressingthem against the aluminum bumps and, through a combination of heat,pressure and time, all of the leads are bonded at one time. The thermodetemperature is preferably maintained between 550° C.-700° C. with adwell time of 0.3 to 0.9 seconds. The anvil temperature is maintained at250° C. It is preferred that the pressure exerted between the bump andinner lead approximate 45,000 psi during bonding, but this may varybetween 31,000 to 59,000 psi. It should be understood that shoulder 30and 32 of bump 14 can either extend significantly past the extent ofcontact land 12 or be smaller than contact land 12. In both cases, it isthe thickness of quartz layer 16 which prevents cracking that mightotherwise occur under the conditions above-described.

Referring now to FIG. 3, a section of an inner lead 50 is shownsubsequent to thermal compression bonding to metal bump 14. The amountof travel between the anvil and the thermode to achieve bonding isadjusted so that the bump 14 is flattened by approximately 30%-40% ofits original height (6 microns +/-2 microns). As a result, the bumpsbecome smooth at the top and expand laterally at the bottom, and thegold layer of lead 50 bonds with gold layer 44 of the bump 14, forming astrong interface. Under the conditions described, a bond strength of50-80 grams per lead is easily achieved.

A significant consequence of causing the bump to extend over shoulderareas 30 and 32 of quartz layer 16, is the possible susceptibility ofquartz layer 16 to cracking during thermal compression bonding. It hasbeen found that as long as the deformation of the aluminum bump does notexceed 40%, the expanding of the aluminum bump over a thick quartzshoulder (greater than 3 microns in thickness) acts as a stress cushionso that the deflection of quartz layer 16 is negligible. If quartz layer16 is too thin, under these conditions, it will crack and will causepotential device failure.

A consequence of the extension of bumps over quartz layer 16 is theexposure of underlying semiconductor structures to the heat and stressof thermal-compression bonding. It has been found that if activesemiconductor structures are present under areas 52 or 54 of thecollapsed portion of aluminum bump 14 or under the contact area betweenbump 14 and contact land 12, that damage can occur to underlying activestructures. In this regard, FIG. 4 shows a plurality of bumps 60, 62,etc. around the periphery of a semiconductor chip 64 (only a corner ofthe chip is shown). The chip contact land areas lying beneath each ofbumps 60 and 62 are connected via conductive pathways 66 to area 68which contains the active semiconductor devices. A band area 70 isprovided between bumps 60, 62, etc. wherein no active circuits arepresent, except conducting pathways 66. Thus, thermal compressionbonding is prevented from damaging underlying active circuits, due totheir absence from band area 70. It has been determined that if thebumps are separated from the active devices by a border of 20 microns orgreater (in any direction), then the effects of thermal compressionbonding on the active devices is negligible. Obviously, if a method ofinterconnection is chosen wherein a tape lead is bonded to pad bymelting a portion of the bump and the tape lead material at a relativelylow temperature, the affects of stress and temperature on deviceparameters are negligible.

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. Accordingly, the present invention is intended to embrace allsuch alternatives, modifications and variances which fall within thescope of the appended claims.

We claim:
 1. A bond structure affixed to a conductive pad on asemiconductor chip, said bond structure comprising:a glassy passivatinglayer having a thickness of at least approximately 3 microns over saidchip, said passivating layer defining an aperture exposing a portion ofsaid conductive pad; and a deformable metal bump having a predeterminedinitial height and covering the portion of said conductive pad exposedin said aperture and extending over an edge of said glassy passivatinglayer and deformed so that its height is reduced by approximately 30% to40% of said predetermined initial height, to thereby expand and furtherextend over said glassy passivating layer.
 2. The structure as recitedin claim 1 wherein, no active device structures are positioned in saidsemiconductor chip beneath portions of said deformed metal bump thatextend over said edge.
 3. The structure as recited in claim 2 furthercomprising:a layer of chromium interposed between said metal bump andsaid exposed conductive pad and covering said edge of said glassypassivating layer.
 4. The structure as recited in claim 3 wherein saidglassy passivating layer is sputtered quartz.
 5. The structure asrecited in claim 4 wherein said quartz has a thickness in the range of 3to 4 microns.
 6. The structure as recited in claim 5 wherein said metalbump includes an aluminum pedestal bonded to said chromium layer.
 7. Thestructure as recited in claim 6 wherein said aluminum pedestal hasadditional layers of chromium, copper, and gold disposed on itsuppermost surface.
 8. The structure as recited in claim 7 wherein saiddeformable metal bump has a thickness, before deformation, in the rangeof from approximately 16 to 21 microns.